function showDiv(obj){
     $(obj).show();
     center(obj);
     $(window).scroll(function(){
          center(obj);
     });
     $(window).resize(function(){
         center(obj);
     }); 
}

function center(obj){
     var windowWidth = document.documentElement.clientWidth;   
     var windowHeight = document.documentElement.clientHeight;   
     var popupHeight = $(obj).height();   
     var popupWidth = $(obj).width();    
     $(obj).css({   
          "position": "absolute",   
         "top": (windowHeight-popupHeight)/2+$(document).scrollTop(),   
         "left": (windowWidth-popupWidth)/2   
     });  
}